In the production of hydrocarbons from a well in a petroleum bearing underground formation, it is typical, especially in water flood recovery operations, that water is produced with the oil and gas. In water flood operations, water can comprise a significant portion and sometimes a majority of the produced fluids. As the water comprises a large constituent, substantial costs may be avoided by separating water from the stream at the production facility rather than handling the extra volume. In an offshore production operation where space weight and capacity are limited and the costs of transporting the production stream to shore may be significant, the cost savings may be extremely significant.
Water is not the only impurity that must be removed from the production fluids. In sandy formations, particulate solids may be produced in the fluid stream. such sand can cause significant problems in production equipment such as abrasion and wear as well as clogging valves and other process equipment. It is desirable to separate the sand for discharge or reinjection. Clearly a system for separating the water and sand would preferably be small lightweight and efficient as well as provide very clean products that can be disposed of in an environmentally sound manner.
Various equipment is available to separate oil from water such as liquid/liquid hydrocyclones which operate very efficiently and that do not have the weight or space requirements of gravity separation devices. Such hydrocyclones are sometimes coupled with other equipment in a system that provides enhanced operation. For example, U.S. Pat. No. 5,021,165 discloses a hydrocyclone in combination with a flotation device that provides a secondary gravity separation of the underflow from the hydrocyclone to provide additional clarification of the water phase. It should be noted that the patent additionally discloses that the system can include a hydrocyclone of the type for separating solids between the liquid/liquid hydrocyclone and the flotation device.
Various equipment is also known for separating solids from liquids. For example U.S. Pat. No. 3,764,008 illustrates a system for cleaning sand from a production stream using two successive solid/liquid hydrocyclones.
In some cases, however, the production stream is not provided from the well at sufficient pressure to operate the complete system to separate the multiphase stream into its various components. Each hydrocyclone requires a certain pressure drop to efficiently separate the components. In addition, some production streams contain dissolved carbon dioxide which may bubble out under the reduced pressure outside the formation. Such carbon dioxide may permit the formation of scale which could clog subsequent valves and process equipment. Moreover, if it is desirable to reinject the water, the scale must be removed or prevented by chemicals or other means as it may clog interstices in the formation reducing the potential productivity of the well.
Accordingly, it is an object of the present invention to provide a method and apparatus for separating a multiphase fluid stream comprising a heavy phase liquid component, a lighter phase liquid component, and a particulate solids component into a plurality of separately classifiable streams which avoids the above noted drawbacks.
The above and other objects of the invention are achieved by the provision of a method and system which separates at least a portion of a multiphase liquid mixture into at least a lighter phase liquid stream and a heavier phase liquid stream. At least a portion of the heavier phase liquid stream is cyclonically separated into an overflow and an underflow wherein the overflow comprises a relatively greater concentration of lighter phase liquid component compared to the heavier phase liquid stream prior to the cyclonic separation. The pressure of at least a portion of the underflow from the cyclonic separation is elevated to a higher pressure and at least a portion of the higher pressure underflow is separated into at least two streams, where one of the streams comprises a greater concentration of at least one component than the other stream.